Field of the Invention
This invention relates to a rectifier circuit, and in particular, it relates to an active switching rectifier circuit employing MOSFETs with current-based control.
Description of the Related Art
Conventional AC to DC rectifying circuits employ diodes by utilizing the diode's P-N junction single direction conductivity. When a relatively small current passes through the diode, the normal diode will have a 0.7V voltage drop across the diode (or about 0.3V for Schottky type diodes). The voltage drop can be up to 1.0V or higher when a relatively high current, such as 100 A or 200 A, passes through the diode. The power loss over the diode will be 100 W or 200 W or more in such situations. Thus, in high current rectifier applications, such as those encountered in alternators in automobiles and other applications, power loss can be significant. In high current rectifier applications, such as 150 A or higher in a 125° C. environment, the heat generated from diode power loss can greatly reduce the reliability and efficiency of the equipment.
MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), in particular power MOSFETs, have been used in rectifier circuits due to their relatively small internal on resistance. For example, a chip made by Linear Technology, model number LT4320/LT4320-1, is intended to be used to control a diode bridge made of four N-channel MOSFETs. In another example, US 20140129850 (assigned to Linear Technology) shows an active MOSFET bridge circuit; paragraph [0048] of that application describes that the MOSFETs are controlled by comparators or passive circuitry to turn on MOSFETs on and off by detecting the incoming voltage polarity. This type of MOSFET control circuits tend to be complex, and have the disadvantage that voltage detection may become unreliable in applications that involve dynamic inductive or capacitive load.